322 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [YoLYl. 



ims — parial, subcircular i3its placed side by side in like locality in Tetrao 

 obscurns. 



The orbits are more fortunate in the completeness of their bony 

 enclosures — the heavy plate generously extended by the ethmoid to 

 divide these cavities very rarely shows any deficiencies. Of all the 

 crania before us Tetrao ohscurus is the only delinquent in this respect, 

 though no doubt this may occur in others. In it quite a vacuity exists 

 near the middle of this septum. Anteriorly the prefrontal and frontal 

 throw out laterally squamous septa of greater or less completeness, that 

 divide these cavities from the common rhinal space. These plates may 

 coalesce with the processes of the lacrymal, as a rare coincidence, and 

 perchance meet the infraorbital style. The foramina for the passage of 

 the optic nerves and the first pair are, as a rule, singularly circular and 

 distinct, the minor apertures about them enjoying a like individuality. 

 They are noted for their greater patentcy among the Partridges. 



A separate canal is devoted to each olfactory nerve immediately 

 below the orbital vaults, that usually at its outstart from the cranial 

 end has a small opening between it and the one of the opposite side. 

 About the entrance to the otocrane we notice principally a rounded, 

 squamous plate thrown down from above by the mastoid, that is pre- 

 sent in all the Grouse. Just below and within, this segment also devel- 

 ops a sharp spicula of bone, posterior to the tympanic articulation, 

 that evidently serves the purpose of keeping this free ossicle in its 

 socket in certain movements of the jaw. 



Peculiarities of the floor of this cavity have already been described 

 above ; in specimens of Tetrao canadensis, carefully selected for me by 

 Mr. William Brewster, of Cambridge, Mass., and forwarded tq me by 

 Prof. J. A. Allen, of the Museum of Comparative Zoology, also of Cam- 

 bridge, to whom my grateful acknowledgments are due for so many like 

 favors, we find, upon viewing the skull from below, the elevations or con- 

 vexities due to these ellipsoidal and wing like formations, reminding 

 one .of their marked resemblance to the acoustic bullae of the tympanic 

 found among the crania of Felidce. 



The author in his various plates and figures believes he has given suf- 

 ficient life size, as they all are, views of basal and superior aspects of 

 the skuUs of these birds, will not enter here into any needless details 

 lOf measurements. The variation in size iu this respect in Centrocercus 

 has already been dwelt upon ; it is not nearly so marked in other genera. 

 The surface of the skulls above have a rough look caused by many 

 minute depressions and groovelets, then running out to the margins of 

 the orbits cause them in some to be finely serrated. 



The Sharp-tailed Grouse is a unique exception to this, it being a bird 

 of rather a delicate skull with smooth cranial superficies. All save Cen- 

 trocercus possess rather depressed foreheads, apparently due more to a 

 slight tilting upwards of the superior orbital peripheries. The lateral 

 temporal fossae are shallow and scarcely noticeable, the muscles they 

 afford lodgment not being remarkable either for their size or strength. 



